Three methods, one mean

You will write the assumed mean method and the step deviation method as functions. You will check that they agree, and then find the best number to assume.

The idea. Both methods give the exact mean of the class marks. The assumed mean method is $a + \dfrac{\sum fd}{N}$ and the step deviation method is $a + h\dfrac{\sum fu}{N}$.

Your tasks.

1. In assumed_mean, return the mean by the assumed mean method, rounded to $4$ decimals.
2. In step_deviation, return the mean by the step deviation method, rounded to $4$ decimals.
3. Set best_a to the class mark that makes $|\sum fd|$ the smallest.